The present invention relates to a novel method and apparatus for measuring the unsteady sedimentation potential of particles in a suspension comprising: a cell which has at least a portion of the suspension of particles inserted therein; a first electrode and a second electrode disposed within the cell; means for vibrating the cell to accelerate the particles of suspension within the cell; and measuring the amplitude of the unsteady sedimentation potential of the particles across the first and second electrodes, for example, by means of a synchronous demodulator. In particular, the present invention provides for the vibration of the cell at a frequency in the range between 0.0001-50 khz.
Determination of the zeta potential of particles in a suspension is very helpful in controlling the addition of auxiliary agents to influence the flocculation and retention characteristics of particles. The addition of auxiliary agents can substantially influence the zeta potential and it is for this reason that much time has been devoted to methods and apparatuses to be used in determining the zeta potential of colloidal particles.
Many attempts have been made at measuring the zeta potential among them the use of ultrasonic sound to measure the vibration potential between two electrodes, and application of a laser beam and optical measurement thereof to determine the electrophoretic mobility of the migrating particles. The use of ultrasonic sound to measure the vibration potential of particles in suspension is disclosed in U.S. Pat. Nos. 4,294,656, 4,381,674 and 4,497,208.
U.S Pat. No. 4,294,656 provides for a process for measuring zeta potential wherein a portion of the suspension is exposed to an ultrasonic sound field in a measuring cell, the measuring cell having two electrodes which extend into the suspension and are spaced from each other by an odd multiple of half ultrasonic wave lengths of the ultrasonic sound field in the suspension, and generating a signal from the voltage thereby formed between the electrodes which corresponds to the state of charge and determines the addition of auxiliary agent.
U.S. Pat. No. 4,381,674 discloses a method of detecting and identifying particulates in the recycling fluid flow of an oil recovery system by counting the number of ultrasonic pulses reflected from the particulates and comparing the number counted with the amount of ultrasonic energy across the flow.
U.S. Pat. No. 4,497,208 discloses a method and apparatus for measuring electro-kinetic properties of charged particles dispersed in a liquid medium which comprises the step of positioning two electrodes to contact the liquid medium, energizing the electrodes with an alternating electrical potential to cause a charged separation between the surfaces of the dispersed particles and the charged layers which surround the particles in the liquid medium and thereby to generate an acoustic signal, spacing an acoustic transducer from the electrodes for detecting an acoustic signal, and measuring the amplitude of the detected signal, the amplitude of the detected signal apparently being a function of the electro-kinetic properties of the particles present in the liquid medium, the number of particles per unit volume and the amplitude of the excitation potential on the electrodes.
Another means of measuring the zeta potential of colloidal particles is described in U.S. Pat. No. 4,046,667 which provides for a microelectrophoresis apparatus for measuring the zeta potential or electrophoretic mobility of particles suspended in a bulk medium, e.g. colloids suspended in a liquid. It further provides for the use of a light beam, microscope and objective lens system for physically determining the zeta potential of colloidal particles. The use of the microscope and requires a highly trained technician to physically determine the zeta potential on a periodic basis. This system is inherently subject to human error and also requires prolonged analysis prior to each measurement.
The aforementioned patents relate either to the measuring of a vibration potential by use of ultrasonic sound in a frequency range of above 100 khz, or to the measurement of electrophoretic mobility by the use of optical methods. Ultrasonic methods which measure a so-called "vibration potential" suffer from a lack of adequate theory linking vibration potential measurements to familiar colloidal properties such as zeta potential. The optical methods suffer from complexity and human error in the measurement of electrophoretic mobility.
It is known that the application of an electric field causes charged particles in a slurry to migrate. Conversely, the migration of charged particles under the action of a body force, such as gravity (this is called sedimentation), will result in the creation of an electric field. The potential difference which arises between separate points in such a system is called the sedimentation potential.
Attempts have been made to measure the sedimentation potential but most are disappointing because steady forces, such as gravity, produce extremely small, steady sedimentation potentials. Small, steady sedimentation potentials are hard to detect due to the occurrence of drift in the electronics, and due to assymmetry potentials which arise at the electrodes which must be introduced into the liquid for purposes of measurement.
The application of unsteady (time-periodic) forces to colloidal particles has been limited historically to ultrasound frequencies above 100 khz as demonstrated in U.S. Pat. Nos. 4,294,656, 4,381,674 and 4,497,208. The principles underlining the aforementioned patents suggests that the colloidal particles are induced to vibrate relative to the liquid due to an interaction with ultrasonic sound waves which are applied to the slurry. The particles vibrate in sympathy with the passage of the ultrasound wave because the sound speed inside the particle is different from the sound speed in the surrounding liquid. An electric potential will exist between two electrodes placed one half wave length apart in the slurry, and this potential is reportedly linked to the zeta potential of the particles. However, no satisfactory theoretical model exists to predict the exact relationship between this acoustic vibration potential measured and zeta potential. Thus, the accuracy of determining the zeta potential using ultrasonic sound frequencies has been found to be unreliable and theoretically unproven.
The present inventor has devised a novel method and apparatus which overcomes the deficiencies of the prior art. Moreover, the present invention provides a method and apparatus for detecting the unsteady sedimentation potential of particles in a suspension which is directly proportionally to the zeta potential of the colloidal particles.
The advantages of the method and apparatus according to the present invention will be further described below.